A listener wearing a conventional hearing-aid demonstrates a substantial reduction in his or her sound externalisation and sound spatialisation abilities and this, in turn, significantly reduces the listener's ability to parse sounds of interest from competing background sounds. On the other hand, a non-hearing impaired listener relies on spatial hearing to separate competing sounds based on the different spatial locations between the sources of the sounds and the listener. Sound spatialisation also assists listeners to focus attention on sounds of interest.
Human spatial hearing relies on the integration of acoustic information from both ears. This acoustic information consists of the binaural difference in the intensity and time of arrival of sound between the two ears and also the monaural spectral cues that result from the location-dependent acoustic filtering of sound by the outer ear. The perception of externalised sounds (i.e., sounds that are heard as outside of the head) relies primarily on the monaural spectral cues provided by the acoustic filtering of the outer ear. Sounds without these spectral cues, but with a consistent interaural time difference cue and interaural intensity difference cue, are perceived as lateralised and inside of the head.
A hearing-impaired listener usually suffers greater hearing loss at higher frequencies. However, due to the shape and size of the outer ear, the frequency range over which the monaural spectral cues play an important role for spatial acuity is generally from about 5 kHz to 20 kHz, which is in the higher range of auditory frequencies. As a result, auditory spatialisation is significantly impaired for the hearing-impaired listener, which ultimately leads to the inability to separate information from background noise. Furthermore, it is the high frequencies above about 8 kHz that are required for accurate spatialisation of speech stimuli.
Various methods for enhancing the spatial hearing of listeners wearing hearing aids have been proposed. One of these methods for enhancing the spatial hearing of listeners wearing hearing aids involves the use of miniature, completely-in-the-canal (CIC) hearing aids to avoid interference with the acoustic filtering of the outer ear. The electronics for the CIC hearing-aids are contained within a small mould that is completely contained within the auditory canal.
Another method for enhancing the spatial hearing of listeners wearing hearing aids involves the use of open or non-occluding ear moulds that do not distort the low-frequency interaural time difference cues.
Yet another method for enhancing the spatial hearing of listeners wearing hearing aids involves adjusting the gains of the left and right hearing aids based on empirical localisation tests in an attempt to preserve the interaural intensity difference cues.
One disadvantage of all of these methods is that they do not use signal processing to enhance and provide high-frequency monaural spectral cues that vary consistently with the location of the sound in space.
Another disadvantage of all of these methods is that they do not make the very high frequency spectral cues (greater than about 8 kHz) more audible.
Terms related to this invention are defined below:
The term “speech frequency band” is the frequency range (approximately, but not exactly, 200 Hz to 4 kHz) that is empirically most important for a listener's speech perception. It may vary slightly from listener to listener and may be determined empirically and/or analytically.
The term “high-frequency band” refers to the frequency band above the speech frequency band.
The term “high frequency component” refers to a frequency component of a sound that occurs in the high frequency band.